Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into electrical power. A horizontal-axis wind turbine generally includes a tower, a nacelle located at the apex of the tower, and a rotor having a plurality of blades and supported in the nacelle by a shaft. The shaft couples the rotor either directly or indirectly with a generator, which is housed inside the nacelle. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator.
Horizontal-axis wind turbines may be anchored on land by securing a lower portion, such as a lower tower flange, of the wind turbine tower to a foundation that extends into the ground. Foundations are designed to far exceed the life expectancy of the various wind turbine components, since replacement or repair is expensive and/or difficult. Conventional foundations include steel-reinforced concrete structures arranged within an excavation pit. The structure includes a centrally positioned steel anchor cage that is generally cylindrical and includes upper and lower annular steel flanges arranged horizontally, and a plurality of high-strength steel anchor bolts extending vertically between the flanges.
Securing the tower of the wind turbine to the anchor bolts causes the anchor bolts to be placed under increased stress. However, like many structural components, the anchor bolts are conservatively designed with a built-in factor of safety designed to withstand such stresses. However, if the anchor bolts do not maintain sufficient post-tensioning, the anchor bolts may begin to loosen. As used herein, post tensioning means to maintain the foundation under high compression, thereby enabling the foundation to suitably withstand various forces and moments exerted by the wind turbine during operation.
A period of time after the wind turbine is installed and operational (for example, three or four years), inspectors and/or operators of the wind turbine may detect that one or more of the anchor bolts is no longer sufficiently post-tensioned. Insufficient post-tensioning may cause the anchor bolts to move within the concrete foundation. This may be visually apparent through cracks propagating in the concrete foundation over time. Left unchecked, if the anchor bolts loosen to below a certain post-tensioning value, the resulting weakening of the concrete foundation may reduce of the life expectancy of the concrete foundation supporting the wind turbine.
For example, with sufficient weakening, the life expectancy of the foundation may be on the order of the life expectancy of the wind turbine itself. Thus, there may be some instances where the wind turbine foundation will have to be repaired or replaced to realize the full life expectancy of the wind turbine. However, repairing or replacing the foundation after wind turbine assembly and operation is costly and time-consuming, and cannot generally be performed while the wind turbine is actively operating (where the blades are actively rotating) or in standstill mode (where the blades are not rotating). Instead, repairing or replacing the foundation after wind turbine assembly currently requires that the tower be removed from the foundation.
Accordingly, there is a need for an improved method of repairing a wind turbine foundation. More particularly, there is a need for an improved method of replacing the one or more anchor bolts of the wind turbine foundation after the wind turbine is assembled, without the need for time-consuming or costly disassembly of the wind turbine.